CODEWITHSUNDEEP
javapolymorphismoverridingsubclassing
ria
ria

Reputation: 7974

Why can't you reduce the visibility of a method in a Java subclass?

Why does the compiler give an error message when you reduce the visibility of a method while overriding it in the subclass?

Upvotes: 43

Views: 13236

Answers (3)

sepp2k
sepp2k

Reputation: 370132

Because if this was allowed, the following situation would be possible:

Class Sub inherits from class Parent. Parent has a public method foo, Sub makes that method private. Now the following code would compile fine, because the declared type of bar is Parent:

Parent bar = new Sub();
bar.foo();

However it is not clear how this should behave. One possibility would be to let it cause a runtime error. Another would be to simply allow it, which would make it possible to call a private method from outside, by just casting to the parent class. Neither of those alternatives are acceptable, so it is not allowed.

Upvotes: 19

Joachim Sauer
Joachim Sauer

Reputation: 308031

Because every instance of the subclass still needs to be a valid instance of the base class (see Liskov substitution principle).

If the subclass suddenly has lost one property of the base class (namely a public method for example) then it would no longer be a valid substitute for the base class.

Upvotes: 66

z -
z -

Reputation: 7168

Because subtypes have to be usable as instances of their supertype.

Upvotes: 1

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